This invention relates to the manufacture of curved glass. More especially the invention relates to a method and apparatus for manufacturing curved glass sheets which are to be used in the manufacture of vehicle windscreens. In particular the invention relates to a method and apparatus for sag-bending pairs of glass sheets which are to be laminated together to produce a vehicle windscreen.
It is known to produce curved glass, such as a single glass sheet for use as a vehicle windscreen, or a pair of glass sheets which are to be laminated together to produce a vehicle windscreen, by supporting the glass on a sag bending mould which is passed through a furnace where it is heated to bending temperature and sags to conform to the shape of the mould.
It has been customary to mount each mould on a carriage which may include a box in which the mould is located, to load each mould with one or a pair of glass sheets at a loading/unloading station and then to advance a succession of loaded carriages through heating sections of a tunnel furnace having roof heaters. Each carriage is stationary at each heating section for a pre-set time. The glass is gradually heated to a sag bending temperature, for example 590.degree. C. to 610.degree. C., by the time it reaches the hottest section of the furnace, where the glass sags to the shape of the mould and attains a required configuration before being removed for cooling. Cooling continues as the carriage is transferred back to the loading/unloading station, where the cooled bent glass is removed and glass to be bent is loaded on to the mould.
The carriages are circulated step-wise around a continuous path which includes the tunnel furnace. This path may be in the form of a horizontal loop around which the carriages are carried back to the loading/unloading station. In other arrangements the carriages removed from the hot end of the furnace may be raised by a lift to a return run above the furnace roof and then lowered to the unloading/loading station, or the carriages may be lowered from the hot end of the furnace to a return run beneath the furnace and then lifted back to the loading/unloading station.
Furnaces of this kind are described in GB-A-No. 1 299 384, GB-A-No. 1 310 670and EP-A-No. 0 132 701.
In the manufacture of laminated windscreens for vehicles flat glass sheets are cut to the required external shape to suit the styling of the vehicle. These glass sheets are then loaded in pairs on to sag bending moulds at the loading/unloading station. Each sag bending mould usually has lateral members with upper curved edges shaped to determine the final curvature to which the glass sheets sag. When the end parts of the sheets are to have a substantial curvature to match the styling of a vehicle, the mould has pivoted wing pieces which pivot upwardly under the influence of the weight of the sagging glass to conform the ends of the glass to the required windscreen shape.
When the cold flat glass is loaded on to the sag bending mould the glass has a limited number of points of contact with the mould, notably the upper ends of the lateral mould members and the predominant points of any wing pieces. It has been observed that these points of contact are points where damage can occur to the lower glass surface. In general these points of contact lie near to the edges of the glass, and can cause breakage during the glass bending operation or during subsequent processing of the glass. Even if breakage does not occur there may be unacceptable flaws in the glass surface. The damage is due to a combination of sliding contact between the metal mould and the glass surface, causing minor scratching, and tensile stresses induced in the glass surface by the thermal gradient between the generally warm glass and the cold spot caused by localised contact with the mould. The tensile stresses tend to cause propagation of cracks from the minor scratches. Further damage may occur at other points as the hot glass sags onto the colder mould during the final stages of the bending.
This invention is based on the discovery that reduction in the temperature differential between the mould members and the glass during heating of the glass, particularly the initial stages of heating of the glass, results in a reduced temperature differential being maintained throughout the sag bending process, thereby minimizing the possibility of damage to the glass surface where it contacts the mould members.